The use of thick-film resistors, capacitors, etc. parts in microcircuits is becoming of increasing importance in the electrical and electronic field. These thick-film components which comprise a layer of ink or paste which may be conductive, partially conductive, semiconductive or nonconductive in nature are deposited on a ceramic substrate by a process which is similar in nature to the silk-screening method whereby a pattern of films is laid down to form conductors, dielectrics, resistors, capacitors or semiconductors. Following the deposition of the film on the substrate, the resulting material is then fired to a temperature usually ranging from about 500.degree. to about 1000.degree. C. or more in air whereby the film is firmly affixed to the substrate. The resultant paste or ink substrate combination can form a microcircuit of passive components and, in addition, if so desired, discrete active components such as transistors or integrated circuit chips can be attached separately to form a thick-film hybrid device.
As hereinbefore set forth, the use of thick-film items or products is becoming more important due to the advantages which these items offer over other technology such as discrete parts, printed circuits, thin films, etc. For example, the designs which are used which have thick-film networks are easy, quick and flexible with low development costs and offer the design freedom and variety of parameter values which are normally available with discrete parts. Furthermore, circuits formed from thick films can combine many types of components such as high value capacitors, resistors, etc., which are not possible with monolithic products. In addition, the method of preparation of thick-film devices is simple inasmuch as the screen printing and heating processes are easy to control and automate. This is in contradistinction to thin-film networks which require a great degree of care in the sputtering and evaporating processes. The operation advantages which are possible when utilizing thick-film devices include high reliability which results from the use of fewer interconnection points. Furthermore, in contrast with discrete parts, the thick-film devices have improved resistance matching and temperature tracking capabilities.
All of the above-enumerated advantages will permit the use of thick-film devices in consumer radio and television products as well as in computers and in industrial electronic devices. These thick-film devices such as resistor networks may be used to replace the carbon resistor while hybrid modules including a thick-film device may be used in television circuits for the horizontal and vertical oscillators, high-voltage dividers and chroma signal processors. Additional uses for these devices are found in telephones, two-way radios, multiplexers, insulators, voltage regulators and heating aids. Likewise, these devices may also be used in industrial control systems such as analog-to-digital and digital-to-analog converters, operation amplifiers, servo amplifiers, power amplifiers and power supply regulators, while in the automotive field hybrid thick-film devices may be used in fuel injection systems. It is thus readily apparent that thick-film devices find a wide variety of uses in many fields.
The silk-screen conductor pastes which are currently in use are produced by combining a noble metal pigment such as gold, silver, platinum, palladium, etc., with a powder glass mixture, an organic vehicle and an organic binder. Thereafter, the paste is silk-screened onto a ceramic substrate and thereafter taken through a firing cycle at a temperature in the range hereinbefore set forth which first burns off the organic vehicle and thereafter melts the glass frit. On cooling, the product is a distribution of metal pigment in a glassy matrix which possesses an electrical conductivity sufficient to produce minimal and predictable resistance in the electric circuit.
The most common commercial inks which are used in thick-film circuitry, as hereinbefore set forth, are based on the noble metals. However, due to the volatility of the prices of these noble metals as well as the availability thereof, there is a strong incentive to replace the noble metal pigments with a nonnoble metal pigment which would result in less costly conductors. While several nonnoble metal systems are currently in use, there are certain disadvantages inherent in these systems which prevent a wide acceptance of the system. One drawback in using these nonnoble conductive metals such as nickel or copper has been that these metals are subject to a relatively ready oxidation of the metal, thereby reducing the conductivity of the metal to a point where it is insufficient in conductive properties to be useful in microcircuits. Another disadvantage is that the inks or conductive surfaces possess a poor solderability. Currently, copper-based systems do possess good electrical conductivity. However, the firing of the ink must be accomplished either in an inert atmosphere or at a low temperature such as a maximum of 700.degree. C. To overcome this relatively low temperature firing, it has been necessary to incorporate a selective oxidatable material into the ink. U.S. Pat. No. 4,122,232 discloses a paste which is used in forming a base metal thick-film electrical conductor which comprises mixing the paste consisting of a base metal powder such as nickel, copper, cobalt or mixtures thereof with a boron powder and a vehicle and a glass frit, said vehicle comprising an organic compound of the type well known in the art. A somewhat similar conductive ink is disclosed in U.S. Pat. No. 4,322,316 which discloses a thick-film conductor paste consisting of boron, copper oxide, and a glass frit as well as an inert vehicle.
Various U.S. patents have shown different inks. For example, U.S. Pat. No. 3,663,276 deals with inks which are used as resistors having a resistance greater than 100,000 ohms per square. However, this reference uses noble metals or noble metal oxides with nonnoble metals of given concentrations. The nonnoble metals oxidize upon firing, thus becoming nonconductive in nature and providing the desired high resistivity. Other U.S. Pat Nos. such as 3,843,379, 3,811,906 and 3,374,110 describe utilizing a noble metal that is mixed with a vitreous frit, an organic binder, a solvent and is thereafter fired in an air atmosphere at an elevated temperature. These patents describe the use of noble metals such as gold, silver, palladium or mixtures thereof. As will hereinafter be shown in greater detail, the process of the present invention uses a nonnoble metal alloy that can be air-fired under elevated temperatures, thus permitting the oxidation of the oxidizable material in preference to the nonnoble metals under the conditions of firing. While certain U.S. Pat. Nos. such as 3,647,532 and 2,993,815 describe the use of nonnoble metals as conductive inks, it is necessary that these inks utilize a furnace with an accurately controlled special type atmosphere. For example, in the former patent, the firing is effected in an essentially neutral or inert atmosphere, except that it contains sufficient oxygen and claims that the upper limit of the oxygen which is present is 0.1% by volume. Further, this reference also utilizes a reducing agent within the ink such as hydrazine hydrate which when decomposed at elevated temperatures releases hydrogen and reacts with excess oxygen, thus preventing oxidation of the base matter in the essentially neutral atmosphere. The purpose of the low oxygen content in this patent is to burn off the binder, but it cannot be any higher inasmuch as it will oxidize the conductive metal and render the ink electrically nonconductive. By utilizing this inert or essentially neutral atmosphere, the atmosphere is identical to a rare gas such as neon, argon, krypton, xenon, radon, etc., which show practically no tendency to combine with other elements. Therefore, an inert atmosphere is neither oxidizing nor reducing which is in contradistinction, as hereinafter set forth in greater detail, to the oxidizing atmosphere of the present invention. U.S. Pat. No. 2,993,815, hereinabove cited, uses two firing operations. The first firing is effected in an air, oxygen or mixed oxygen and inert gas environment so as to form the glass-metal bond. Following this, the second firing is effected in a reducing atmosphere possessing a critical composition of nitrogen, hydrogen and small amounts of oxygen to reduce the oxidized metal. Nonnoble metals such as copper, nickel, alloys of nickel and copper or iron when fired in an air atmosphere at 840.degree. C. are known to oxidize rapidly and therefore will no longer be able to be utilized as conductive metals.
It is also known that reducing agents can be added to the glass frit. However, this produces spotty conduction zones. The addition of antimony, chromium, charcoal or other oxygen scavengers can be mixed or blended into the conductive ink, but on firing, reduction is nonuniform and will tend to occur only where the oxygen scavenger is present. U.S. Pat. No. 3,711,428 describes the mixing of charcoal with the ink. However, this action is taken to prevent blistering or cratering of the resistor, the charcoal burning off and thus leaving the metal exposed for oxidation. While this does not cause problems for the noble metal, there is substantial oxidation of nonnoble metals such as copper. Another U.S. Pat. No. 2,795,680, utilizes a ceramic base to which is bonded a cross-linked epoxy resin and a conductive and nonconductive powder. The resin is cross-linked at 250.degree. C. which is well below the firing temperature which is utilized in the present invention. In the event that resistors need to be cofired, the conductor ink could not withstand the higher temperature.
U.S. Pat. No. 3,943,168 discloses conductor compositions comprising nickel borides in which the compositions are finely divided inorganic powders comprising one or more compounds of nickel such as a mixture of nickel boride and nickel boride-silicide. It is also stated in this patent that the compositions may contain nickel metal powder in which the nickel powder may comprise up to 8% of the total weight of the nickel and nickel compounds present. Likewise, U.S. Pat. No. 4,130,854 discloses a borate-treated nickel pigment for metalizing ceramics, the borate coating forming a glass on the surface of the nickel powder, the borate forming an oxidation resistant film which aids in the adhesion of the nickel to the substrate.
In addition to the aforementioned U.S. patents, two other U.S. patents disclose conductive metal pigments. U.S. Pat. No. 4,079,156 describes a conductive metal pigment which is prepared by alloying a nonnoble conductive metal with an oxidizable material followed by mixing the resulting alloy with a vitreous frit and an organic vehicle to form an ink. The ink is then screened onto a substrate followed by firing in an oxidizing atmosphere for a period of time sufficient to oxidize the oxidizable material without oxidation of the nonnoble metal. The oxidizable material is present in the alloy in an amount within the range of from about 0.1 to about 10% by weight of the alloy. Likewise, U.S. Pat. No. 4,388,347 describes a conductive pigment-coated surface in which a nonnoble conductive metal is alloyed with from about 12% to about 25% by weight of an oxidizable material, mixing the alloy with an organic vehicle to form an ink and thereafter following the same steps as shown in the previous patent to produce a conductive pigment-coated surface. In addition, a vitreous frit containing mixtures of oxides of various metals such as silicon, calcium, sodium, magnesium, iron, zinc, tin, etc. may also be present in the conductive pigment. The presence of a frit in the conductive pigment differs greatly from the lead-containing compound of the present invention inasmuch as the glass frit is a noncrystalline or amorphous material which possesses little or no long-range atomic order. In contradistinction to this, the lead-containing compounds such as lead oxide, etc. are crystalline materials with a definite atomic order and cannot be considered frits.
The inks or conductive pigment-coated surfaces which have been previously mentioned in the discussion of previous U.S. patents do possess another disadvantage in that the inks are attacked to some degree by water. The susceptibility to moisture is due in all probability to the water solubility of the boron oxide glass which is produced during the firing step of the process. As will hereinafter be shown in greater detail, it has now been discovered that, by adding a lead-containing compound of the type hereinafter set forth, it is possible to obtain improved physical and electrical characteristics of the conductive pigment-coated surface produced by the process of the present invention.